1. Field of the Invention
The present invention relates to a switching device and a network adapter for efficiently transmitting voice-band signals on an ATM network, and a multiplexer and a demultiplexer forming principal parts thereof.
2. Background of the Invention
&lt;Background of ATM&gt;
ISDN (integrated service digital network) is a communication network for integratedly providing various communication services such as voice communication and data communication through generic user-network interfaces. This ISDN is now developing to B-ISDN (broadband aspects of ISDN) aimed at providing high-speed communication services such as a visual telephone, a picturephone meeting and a cable television and simultaneously and efficiently providing a number of services which are varied with media. An ATM (asynchronous transfer mode) is a key technique supporting the B-ISDN. A network generally employing the ATM technique including the B-ISDN is generally called an ATM network.
In the ATM, blocks (called "ATM cells" or "cells") which are provided with header parts storing labels are transmitted in response to occurrence of information, similarly to a packet mode in ISDN which is precedent to the B-ISDN, i.e., N-ISDN (narrowband aspects of ISDN). Thus, it is possible to cope with arbitrary communication speeds including variable speed communication (service system transmitting burstly generated information at its original variable speed).
In the ATM, further, the cells are at a fixed length, dissimilarly to the packet mode. Therefore, the communication can be made with no inhibition of speed increase by positional detection for the header parts. Further, all data are stored in cells of the same shapes and subjected to processing such as exchange regardless of communication media such as voices, images and data, whereby it is possible to transmit various types of data in a single network in a one-way manner regardless of differences between the media.
In addition, protocols are simplified in the ATM with the background of quality improvement in the transmission media, while the same can be processed in a hardware manner with no software. Namely, the ATM is a technique which is mainly adapted to solve the problem of difficulty in coping with speed increase in the packet mode.
&lt;Principle of ATM&gt;
As hereinabove described, all data are carried on cells in the ATM. FIG. 16 is a structural diagram showing the structure of a cell 13. The data length of the cell 13 is fixed at 53 octets (1 octet=8 bits). The cell 13 is formed by a header part (cell header) 14 of 5 head octets and an information field part (also referred to as a "payload") 10 of 48 octets. The header part 14 stores an identifier indicating the destination, so that the cell 13 is transmitted to the destination through the identifier. Information such as a voice, an image and data is stored in the information field part 10.
FIG. 17 is a block diagram showing an exemplary structure of paths for transmitting cells each having the aforementioned structure, i.e., an ATM network. As shown in FIG. 17, two types of hierarchical information transmission paths (connections) including virtual channels (VC) 23a to 23d and virtual paths (VP) 24a to 24c and 25a to 25h are logically defined in the ATM network. The VCs 23a to 23d are connections forming the basis of ATM services, and a series of cells storing information such as voices and data of subscribers 21a to 21h are identified as those belonging to a single VC. In general, each VC is set between a pair of subscribers, as illustrated in FIG. 17.
The ATM network identifies the VC for a certain cell through a VCI (virtual channel identifier) which is stored in each header part. The values of such VCIs may be reloaded by ATM switching nodes (relay points) 22a to 22c. When a VC is set, a relevant switching node holds a conversion table of the VCI, so that connection is theoretically maintained between end subscribers. In such VC connections, respective segments which are delimited by the switching nodes are called VC links. Therefore, a single VCI value can be regarded as being supplied to a single VC link.
On the other hand, the VPs, each of which is formed by bundling a plurality of VCs, are generally set between the subscribers and the switching nodes (VCPs 25a to 25h in FIG. 17) or pairs of switching nodes (VPs 24a to 24c in FIG. 17). The ATM network identifies a VP for a certain cell through a VPI (virtual path identifier) which is stored in each header part. The values of such VPIs are released by the ATM switching nodes. Namely, the ATM switching nodes are nodes between the VC links, as well as terminating ends of the VP connections.
As to a system of transmitting voice-band signals through the ATM network, currently defined is a method of transmitting a 1 octet voice signal every 125 .mu.sec. with a 1 octet control signal, while storing the same in an information field part of a single cell. FIG. 18 typically shows the principle of this method. Voice signals 12 which are inputted in the unit of 1 octet every 125 .mu.sec. are stored in cells 13 by a cellularizing unit 11, and transmitted every 125 .mu.sec. These cells 13 are transmitted through the ATM network, as hereinabove described.
Each cell 13 stores a 1 octet control signal 15 and a 1 octet voice signal 12 in its information field part 10. In the information field part 10 of 48 octets, therefore, only 2 octets are used with no employment of the remaining 46 octets. Namely, the network bands are disadvantageously used in vain in transmission of voice signals through the ATM network.
In order to solve this problem, it may be possible to store the voice signals of the cells 13 belonging to a single VC by the information field length for simultaneously storing the same in a single cell. However, storage of the voice signals is accompanied with corresponding transmission delay. When 47 octet voice signals and 1 octet control data are stored in a single cell, for example, the delay reaches 5.875 msec. An efficient real-time operation is required for the voice signal transmission service, and the delay caused by this method results in unneglectable reduction in transmission quality.